BACKGROUND: It is uncertain whether sex and race affect thrombogenicity in patients with coronary artery disease. We evaluated the effects of sex and race on thrombogenicity in patients with coronary artery disease treated with aspirin. METHODS AND RESULTS: Patients on aspirin therapy for 1 week or longer with known or suspected coronary artery disease undergoing nonurgent cardiac catheterization (n=1172), of whom 924 were on aspirin and clopidogrel therapy, were studied. The primary end point was thrombin-induced platelet-fibrin clot strength (MAKH) measured by thrombelastography. Secondary end points included coagulation index, a measure of overall coagulation; G, another measure of clot strength; and maximal platelet aggregation. Women had greater MAKH, G, and coagulation index than men, both with and without clopidogrel therapy (with clopidogrel: 68.3±6 versus 65.8±6 mm, P<0.0001; 11.4±3 versus 9.5±4 dyne/cm(2), P<0.0001; and 0.12±3 versus -0.7±3, P=0.003, respectively). Platelet aggregation (induced by ADP, thrombin receptor activating peptide, or collagen) did not differ between sexes. Black patients had greater MAKH and G than white patients (with clopidogrel: 67.8±7 versus 66.4±6 mm, P=0.005; 11±4 versus 10±3 dyne/cm(2), P=0.02, respectively). Black women had the highest MAKH levels. By multivariate analysis, sex, race, diabetes, platelet count, and hemoglobin level were independently associated with MAKH . Sex, but not race, was also associated with the frequency of MAKH ≥72 mm (a threshold related to ischemic event occurrence in patients undergoing coronary intervention). CONCLUSIONS: Sex and race independently influence platelet-fibrin clot strength. Black women appear to have the highest thrombogenicity profile, potentially conferring a high-risk phenotype for thrombotic event occurrence.